These days I try to use "Flag" metal paint, instead of Hammerite: it's not politically correct and uses a formula very similar to the original (good) Hammerite formula. New Hammerite does work, but it hates grease and IMHO is not as tough once it hardens off (which takes weeks).
Double check though that Flag paints haven't given in to correctness though and changed their formula. I have a number of tins of it in nice colours, but haven't actually bought one for a while. They have (had) a web site tho, IIRC it had distributors on it.
E.
PS: Forgot to say - Flag was in Toolstation, but I think they've merged some of their operation with Wickes, and I"m not sure they still stock it. Last time I looked, the more intersting colours weren't available from them any more, only black and white.